1. Field of the Invention
Embodiments of the present invention generally relate to Microwave Integrated Circuits (MIC) and monolithic devices, and more specifically, to transitions between waveguides and microstrips for devices operating in microwave and millimeter wave frequencies.
2. Description of the Background Art
Conventional techniques have been designed and developed to facilitate efficient transitions between waveguide and microstrip structures. These transitions may be used in a variety of integrated circuit devices which may operate in the RF, microwave, and millimeter wave frequency regimes. The transitions can effectively serve to act as a bridge between a front end of a system which transmits and receives electromagnetic (EM) waves, and the signal processing circuitry which may condition, exploit, and/or convert the EM waves into useful signals.
FIG. 15 depicts a conventional transition 1500 having a transition between a waveguide and a microstrip consistent with the conventional art. Device may include a open-ended waveguide 1510, a substrate 1512, a backshort 1514, a microstrip 1516, and a conductor pad 1518.
Open ended waveguide 1510, which has an opening having width A and height B, may either transmit or receive EM waves. The other end of open ended waveguide 1510 may be attached to substrate 1512. Substrate 1512 may have microstrip 1516 and conductor pad 1518 formed thereon. Backshort 1514 may be attached to substrate 1512 on an opposite side opposing open-ended waveguide 1510. As shown here, backshort 1514 can be a closed-ended waveguide having a length at least a quarter wavelength (λ/4) of the EM wave. For the conventional device, the long length of backshort 1514 is desired for proper operation of the conventional transition, which is described briefly below.
In one example, an incoming EM wave may be received at the open end of open-ended waveguide 1510, and propagate along its length toward substrate 1512. One portion of the EM wave incident at substrate 1512 may be collected by conductor pad 1518. Another portion of the incident EM wave may pass through substrate 1512 and be reflected off the closed end of backshort 1514. The reflected wave may travel back toward conductor pad 1518, and be collected thereon. Because the length of the conventional backshort 1512 may be λ/4 or longer, the reflected wave may combine in phase at conductor pad 1518 with the incident EM wave. The combine wave may then induce a current at conductor pad 1518 which may be conducted along microstrip 1516.
FIG. 16 depicts an equivalent circuit 1600 which may model conventional transition 1500 (of FIG. 15). A first sub-circuit 1610 models open-ended waveguide 1510, having a characteristic impedance Z1. A second sub-circuit 1616 models microstrip 1516, having a characteristic impedance Z2. It may be desirable to provide a matching circuit 1614 to connect each equivalent sub-circuit so that power transfer may be maximized. It also may also be desirable to optimize the parameters of open ended waveguide 1510 and microstrip 1516 to design matching circuit 1614, so that the EM energy input from open-ended waveguide 1510 is properly convened into microstrip 1516.
One potential issue with conventional transition 1500 is that it may be difficult to match the impedance between open-ended waveguide 1510 and microstrip 1516 given the large relative difference in the magnitude of their respective impedances. For example, the characteristic impedance of open ended waveguide 1510 for frequencies within the microwave region may usually be approximately 300-500 ohms, and the characteristic impedance of microstrip 1516 for the same frequencies may be 50 ohms. Given the differences in impedances, and the interaction of EM fields within the waveguides, it may be difficult to properly realize matching circuit 1614, which may utilize sophisticated three-dimensional circuit design.
Another potential issue with conventional transition 1500 may be the constraint that backshort 1514 has a considerable length which typically is greater than λ/4. This is driven by the desirability that backshort 1514 should appear as an “open circuit” from the viewpoint of a-a′ as shown in FIG. 16. The backshort length becomes longer as the frequencies become lower, which may be a significant concern in devices when the frequencies are lower than 10 GHz.
Because the conventional techniques may result in devices having considerable size, they may be unsuitable for applications requiring portable operation. Additionally, conventional devices may be associated with higher cost and reduced reliability due to greater component complexity and increased component numbers.